Our neighbours house (probably, you’d need a plumb line to check) overhangs our garage. But there’s only a 1ft or so soak away (I presume, it’s filled with gravel) between the houses which I’ve no idea who owns as it’s on our side at the front (his house extends to become the boundary wall), but at the back his fence attaches to our garage.
We’re already a 4 bed house though, so extending sideways is probably never going to happen. And even if it did, at most we’d just be building en-suites and wardrobes so going upto the boundary wouldn’t be an issue because we wouldn’t need to.
So it depends on the house, if it’s a 2 bed in a street of 4 bed houses, then yes it might be an issue with future buyers. If it’s a 4 bed in a street of 3 beds, then no one probably want’s it extending to a 5 bed.